This sample chapter from CCIE: Routing TCP/IP Volume I shows how routers can discover how to correctly switch packets to their respective destinations automatically and share that information with other routers via dynamic routing protocols.

This chapter is from the book

This chapter is from the book

The last chapter explained what a router needs to know to correctly switch
packets to their respective destinations and how that information is put into
the route table manually. This chapter shows how routers can discover this
information automatically and share that information with other routers via
dynamic routing protocols. A routing protocol is the language a router
speaks with other routers in order to share information about the reachability
and status of networks.

Dynamic routing protocols not only perform these path determination and route
table update functions but also determine the next-best path if the best path to
a destination becomes unusable. The capability to compensate for topology
changes is the most important advantage dynamic routing offers over static
routing.

Obviously, for communications to occur the communicators must speak the same
language. There are eight major IP routing protocols from which to choose; if
one router speaks RIP and another speaks OSPF, they cannot share routing
information because they are not speaking the same language. Subsequent chapters
examine all the IP routing protocols in current use, and even consider how to
make a router "bilingual," but first it is necessary to explore some
characteristics and issues common to all routing protocolsIP or otherwise.

Routing Protocol Basics

All dynamic routing protocols are built around an algorithm. Generally, an
algorithm is a step-by-step procedure for solving a problem. A routing
algorithm must, at a minimum, specify the following:

A procedure for passing reachability information about networks to other
routers

A procedure for receiving reachability information from other routers

A procedure for determining optimal routes based on the reachability
information it has and for recording this information in a route table

A procedure for reacting to, compensating for, and advertising topology
changes in an internetw_ork

A few issues common to any routing protocol are path determination, metrics,
convergence, and load balancing.

Path Determination

All networks within an internetwork must be connected to a router, and
wherever a router has an interface on a network that interface must have an
address on the network. This address is the originating point for reachability
information.

Figure 4.1 shows
a simple three-router internetwork. Router A knows about networks 192.168.1.0,
192.168.2.0, and 192.168.3.0 because it has interfaces on those networks with
corresponding addresses and appropriate address masks. Likewise, router B knows
about 192.168.3.0, 192.168.4.0, 192.168.5.0, and 192.186.6.0; router C knows
about 192.168.6.0, 192.168.7.0, and 198.168.1.0. Each interface implements the
data link and physical protocols of the network to which it is attached, so
the router also knows the state of the network (up or down).

Figure 4.1 Each
router knows about its directly connected networks from its assigned addresses
and masks.

At first glance, the information-sharing procedure seems simple. Look at
router A:

Router A examines its IP addresses and associated masks and deduces that
it is attached to networks 192.168.1.0, 192.186.2.0, and 192.168.3.0.

Router A enters these networks into its route table, along with some sort
of flag indicating that the networks are directly connected.

Router A places the information into a packet: "My directly
connected networks are 192.168.1.0, 192.186.2.0, and 192.168.3.0."

Router A transmits copies of these route information packets, or
routing updates, to routers B and C.

Routers B and C, having performed the same steps, have sent updates with
their directly connected networks to A. Router A enters the received information
into its route table, along with the source address of the router that sent the
update packet. Router A now knows about all the networks, and it knows the
addresses of the routers to which they are attached.

This procedure does seem quite simple. So why are routing protocols so much
more complicated than this? Look again at Figure
4.1.

What should router A do with the updates from B and C after it has
recorded the information in the route table? Should it, for instance, pass
B's routing information packet to C and pass C's packet to B?

If router A does not forward the updates, information sharing may not be
complete. For instance, if the link between B and C does not exist, those two
routers would not know about each other's networks. Router A must forward
the update information, but this step opens a whole new set of problems.

If router A hears about network 192.168.4.0 from both router B and router
C, which router should be used to reach that network? Are they both valid? Which
one is the best path?

What mechanism will be used to ensure that all routers receive all
routing information while preventing update packets from circulating endlessly
through the internetwork?

The routers share certain directly connected networks (192.168.1.0,
192.168.3.0, and 192.168.6.0). Should the routers still advertise these
networks?

These questions are almost as simplistic as the preceding preliminary
explanation of routing protocols, but they should give you a feel for some of
the issues that contribute to the complexity of the protocols. Each routing
protocol addresses these questions one way or another, as will become clear in
following sections and chapters.

Metrics

When there are multiple routes to the same destination, a router must have a
mechanism for calculating the best path. A metric is a variable assigned
to routes as a means of ranking them from best to worst or from most preferred
to least preferred. Consider the following example of why metrics are needed.

Assuming that information sharing has properly occurred in the internetwork
of Figure 4.1, router
A might have a route table that looks like Table 4.1.

This route table says that the first three networks are
directly connected and that no routing is needed from router A to reach them,
which is correct. The last four networks, according to this table, can be
reached via router B or router C. This information is also correct. But if
network 192.168.7.0 can be reached via either router B or router C, which path
is the preferable path? Metrics are needed to rank the alternatives.

Different routing protocols use different, and sometimes multiple, metrics.
For example, RIP defines the "best" route as the one with

Hop Count

A hop count metric simply counts router hops. For instance, from router A it
is 1 hop to network 192.168.5.0 if packets are sent out interface 192.168.3.1
(through router B) and 2 hops if packets are sent out 192.168.1.1 (through
routers C and B). Assuming hop count is the only metric being applied, the best
route is the one with the fewest hops, in this case, A-B.

But is the A-B link really the best path? If the A-B link is a DS-0 link and
the A-C and C-B links are T-1 links, the 2-hop route may actually be best
because bandwidth plays a role in how efficiently traffic travels through the
network.

Bandwidth

A bandwidth metric would choose a higher-bandwidth path over a
lower-bandwidth link. However, bandwidth by itself still may not be a good
metric. What if one or both of the T1 links are heavily loaded with other
traffic and the 56K link is lightly loaded? Or what if the higher-bandwidth link
also has a higher delay?

Load

This metric reflects the amount of traffic utilizing the links along the
path. The best path is the one with the lowest load.

Unlike hop count and bandwidth, the load on a route changes, and therefore
the metric will change. Care must be taken here. If the metric changes too
frequently, route flappingthe frequent

Delay

Delay is a measure of the time a packet takes to traverse a route. A
routing protocol using delay as a metric would choose the path with the least
delay as the best path. There may be many ways to measure delay. Delay may take
into account not only the delay of the links along the route but also such
factors as router latency and queuing delay. On the other hand, the delay of a
route may be not measured at all; it may be a sum of static quantities defined
for each interface along the path. Each individual delay quantity would be an
estimate based on the type of link to which the interface is connected.

Reliability

Reliability measures the likelihood that the link will fail in some
way and can be either variable or fixed. Examples of variable-reliability
metrics are the number of times a link has failed or the number of errors it has
received within a certain time period. Fixed-reliability metrics are based on
known qualities of a link as determined by the network administrator. The path
with highest reliability would be selected as best.

Cost

This metric is configured by a network administrator to reflect more- or
less-preferred routes. Cost may be defined by any policy or link characteristic
or may reflect the arbitrary judgment of the network administrator.

The term cost is often used as a generic term when speaking of route
choices. For example, "RIP chooses the lowest-cost path based on hop
count." Another generic term is shortest, as in "RIP chooses
the shortest path based on hop count." When used in this context, either
lowest-cost (or highest-cost) and shortest (or
longest) merely refer to a routing protocol's view of paths based on
its specific metrics.

Convergence

A dynamic routing protocol must include a set of procedures for a router to
inform other routers about its directly connected networks, to receive and
process the same information from other routers, and to pass along the
information it receives from other routers. Further, a routing protocol must
define a metric by which best paths may be determined.

A further criteria for routing protocols is that the reachability information
in the route tables of all routers in the internetwork must be consistent. If
router A in Figure 4.1
determines that the best path to network 192.168.5.0 is via router C and if
router C determines that the best path to the same network is through router
A, router A will send packets destined for 192.168.5.0 to C, C will send them
back to A, A will again send them to C, and so on. This continuous circling
of traffic between two or more destinations is referred to as a routing loop.

The process of bringing all route tables to a state of consistency is called
convergence. The time it takes to share information across

Figure 4.2 shows
an internetwork that was converged, but now a topology change has occurred.
The link between the two left-most routers has failed; both routers, being directly
connected, know about the failure from the data link protocol and proceed to
inform their neighbors of the unavailable link. The neighbors update their route
tables accordingly and inform their neighbors, and the process continues until
all routers know about the change.

Figure 4.2 Reconvergence
after a topology change takes time. While the internetwork is in an unconverged
state, routers are susceptible to bad routing information.

Notice that at time t2 the three left-most routers know about the
topology change but the three right-most routers have not yet heard the news.
Those three have old information and will continue to switch packets
accordingly. It is during this intermediate time, when the internetwork is in an
unconverged state, that routing errors may occur. Therefore convergence time is
an important factor in any routing protocol. The faster a network can reconverge
after a topology change, the better.

Load Balancing

Recall from Chapter 3, "Static Routing," that load balancing is the
practice of distributing traffic among multiple paths to the same destination
in order to use bandwidth efficiently. As an example of the usefulness of load
balancing, consider Figure
4.1 again. All the networks in Figure
4.1 are reachable from two paths. If a device on 192.168.2.0 sends a stream
of packets to a device on 192.168.6.0, router A may send them all via router
B or router C. In both cases, the network is 1 hop away. However, sending all
packets on a single route probably is not the most efficient use of available
bandwidth. Instead, load balancing should be implemented to alternate traffic
between the two paths. As noted in Chapter 3, load balancing can be equal cost
or unequal cost and per packet or per destination.